Developing an analytical technique to study the critical zone through lignin phenols in speleothem samples: paleoenvironmental reconstructions during the Early Holocene in the Apuan Alps Region.

Speleothems are valuable archives of past climatic and environmental conditions, and among the proxies they contain, lignin oxidation products (LOPs) have emerged as a promising marker for paleo-vegetation studies (Blyth et al., 2016). LOPs products provide insights into the abundance and type of the source vegetation (Hedges & Mann, 1979). However, speleothems contain only a small fraction of organic matter compared to other terrestrial archives. A new sensitive method for analyzing LOPs in speleothems was proposed by Heidke et al., (2018). My work aimed to assess the potential of LOPs in speleothems as a proxy for paleo-vegetation and improve the methodological approach. We present the first quantitative record of LOPs from the Holocene flowstone RL18 sampled in the Renella Cave, Italy. The methodology was evaluated using blank and test samples, revealing that contamination mainly results from airborne particles rather than steps in the analysis. Yet, the signal-to-noise ratio and reproducibility are fully satisfactory. The LOP analysis in RL18 shows little variability in the C/V and S/V ratios -proxies for the source plant type (Hedges & Mann, 1979)- implying relatively stable vegetational conditions during the flowstone formation, with angiosperm-derived plant material predominating over gymnosperms. A notable peak in the Σ8 signal, i.e. an estimate of the total lignin content, was observed at ca. 80-100 mm depth. Further investigations into correlations with trace element concentrations, stable isotopes, and fluorescence emissions suggest that such peak corresponds to a period of enhanced rainfall and reduced water residence time into the cave, possibly linked to lower soil degradation. Our results highlight the potential of this method for quantifying LOPs in speleothems, even from small samples (<2g), and demonstrate the potential of LOPs as vegetation biomarkers. A multiproxy approach remains essential for a comprehensive understanding of past environments.